Implantable device with optical telemetry

ABSTRACT

A system is provided for optically communicating with an implantable device. In one embodiment, the system includes an implantable device having a large memory and an external unit which downloads information from the memory for analysis and display. The implantable device includes a light-emitting diode (LED) and a modulator for driving the LED. Although various frequencies can be used, frequencies which experience relatively little attenuation through body tissue are presently preferred. The external device includes a photomultiplier tube (PMT) and a demodulator for equalizing and demodulating the detection signal produced by the PMT in response to detected light. A high bandwidth channel (perhaps as much as  500  Mbits/sec) is created by these components. This channel advantageously allows for a substantially reduced download time.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates generally to wireless communicationsystems for devices implanted in the body, and more particularly tooptical communication between an implanted device and a device externalto the body.

[0003] 2. Description of the Related Art

[0004] Implantable devices have become a standard method of treatingvarious medical conditions, many of which relate to the heart. Examplesof devices which are routinely implanted include pacemakers,defibrillators, and nerve stimulators. These devices and others whichhave not yet become routine (such as implanted personal identificationchips) are being provided with large memories for storing vast amountsof data. In the case of medical devices, this data may includephysiological data such as the electrogram (electrical waveform at theelectrodes), instantaneous heart rate, blood pressure, volume pumped,body temperature, etc., and configuration data such as mode ofoperation, amplifier sensitivity, filter bandwidth, and error messages.Often the device stores data that has been collected over a period ofhours or days. This data is periodically retrieved by a doctor tomonitor the patient's condition and to monitor the device's status. Inresponse, the doctor might re-program the device for a different mode ofoperation, sensitivity setting, etc..

[0005] A method is needed to retrieve this data rapidly. The retrievalneeds to be rapid so as to minimize the inconvenience to the patient whowill usually have to remain in the doctor's office for the dataretrieval process. To download four megabytes of medical device data,for example, at 20 Kbit/s would take nearly a half-hour—an undesirablylong time for both the patient and medical professional or technician.

[0006] One method for data retrieval is the use of electromagneticcoupling between a pair of coils. One coil is excited to induce acurrent in the other. Modulation of the excitation signal can bedetected in the induced current, and so communication is achieved. Theproblem with this is bandwidth. The coils each have a self-inductancewhich acts to attenuate high frequency signals, so that the bandwidth ofcommunications is limited.

[0007] Another method for data retrieval is to provide a directelectrical connection. A wire connected to the implanted device ispassed directly through the skin and coupled to the external device.Inherent with this technique is increased discomfort and increased riskof infection.

[0008] Thus, another method is needed to transfer a large amount of dataquickly from the implanted device to the external device with minimaldiscomfort.

SUMMARY OF THE INVENTION

[0009] Accordingly, there is provided herein a system for communicatingbetween an implantable device and an external device. In one embodiment,the system includes an implantable device having a large memory and anexternal unit which downloads information from the memory for analysisand display. The implantable device includes a light-emitting diode(LED) and a modulator for driving the LED. The LED emits a modulatedlight signal representing the data that is stored in memory. One lightfrequency range which may be used is 4.3×10¹⁴-5.0×¹⁴ Hz, as body tissueexhibits good transmission in this range. The external device includes aphoto-multiplier tube (PMT) for detecting and amplifying the modulatedlight signal, and a demodulator for equalizing and demodulating thedetection signal produced by the PMT in response to modulated light.

[0010] These components will support a high bandwidth optical channelcapable of carrying as much as 500 Mbits or more, and thereby providefor a substantially reduced data retrieval time. The implantable devicemay further include a receiver coil which has currents induced inresponse to a communication signal from the external device. A powerconverter may be coupled to the receiver coil to convert the inducedcurrents into energy for powering the LED.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Other objects and advantages of the invention will becomeapparent upon reading the following detailed description and uponreference to the accompanying drawings in which:

[0012]FIG. 1 shows an implantable medical device having opticaltelemetry, implanted in an environment within which a high-bandwidthchannel would be desirably employed;

[0013]FIG. 2 is a block diagram of an implantablepacemaker/defibrillator;

[0014]FIG. 3 is a schematic diagram illustrating communications betweenan implantable device and an external device;

[0015]FIG. 4 is a block diagram of portions of an external device;

[0016]FIG. 5 is a block diagram of a telemetry module which supports anoptical communications link;

[0017]FIG. 6 shows an exemplary configuration of the system; and

[0018]FIG. 7 shows a second exemplary configuration of the system.

[0019] While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexamples in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0020] The following description illustrates the principles of theinvention with respect to an implantable pacemaker (“pacer”) and anexternal device (“programmer”). The invention, however, is directed toan improved telemetry link between any implantable device and anyexternal device configurable to download information from theimplantable device. Thus, the invention applies to implantablecardioverter/defibrillators (ICD's), nerve stimulators, drug deliverydevices, or any other implantable device configured to transmit data toan external device.

[0021] Turning now to the figures, FIG. 1 shows a human torso 102 havinga heart 104 and an implanted pacer 106. Also shown is a wand 108 whichis an extensible portion of an external programmer 110. Wand 108 isplaced on an exterior surface of torso 102 near to the pacer 106. In theembodiment shown, pacer 106 is a pacemaker coupled to heart 104 toassist in regulating its operation. In any case, pacer 106 includes amemory for storing data for later retrieval. In the case of a pacemaker,the data may represent measured physiological signals such as cardiacvoltages (EKG signals), blood temperatures, oxygen levels, sugar levels,etc.

[0022] Illustratively, programmer 110 is a programmer/analyzer for useby a physician. The programmer/analyzer operates to download informationstored in pacer 106 by transmitting signals which place the pacer in amode for downloading, and thereafter detecting signals sent by thedevice. Then, under control of the physician or other medicalprofessional, the programmer/analyzer operates to analyze and displaythe information in a format which allows the physician to diagnose anyproblems. After performing an analysis, the physician may instruct theprogrammer/analyzer to adjust operating parameters of pacer 106. If thisis the case, the programmer/analyzer provides new operating parametersto pacer 106.

[0023]FIG. 2 is a block diagram of an exemplary pacer 106. Pacer 106 hasa power supply 202 coupled to a microprocessor 204. Power supply 202provides support to all the devices shown in FIG. 2 through connectionsnot shown. Microprocessor 204 is coupled to a memory 206, a firstinterval timer 208, and a second interval timer 210 via an I/O(input/output) bus 211. Microprocessor 204 is also coupled to control anatrium sensor/stimulator 212 and a ventricle sensor/stimulator 214, eachof which may be coupled to the heart by flexible leads. Finally,microprocessor 204 is coupled to a telemetry module 218 to communicatewith programmer 106.

[0024] Microprocessor 204 preferably is programmable and operatesaccording to a program stored in a nonvolatile memory. The program oftenis parameterized—i.e. one or more of the operations the microprocessorperforms is alterable by setting a parameter. For example, themicroprocessor may be programmed to periodically trigger atriumstimulator 212. One of the parameters for this operation might be avalue specifying the rate at which the stimulator is triggered. Theparameters may be provided to microprocessor 204 via telemetry module218 and stored in memory 206.

[0025] Pacer 106 in FIG. 2 uses first interval timer 208 to determinethe delay between trigger signals applied to atrium stimulator 212 andventricle stimulator 214. Further, second interval timer 210 measuresthe time since the last heartbeat sensed by the atrium sensor/stimulator212 or ventricle sensor/stimulator 214. When either timer elapses, theelapsed timer asserts an interrupt to microprocessor 204 to notifymicroprocessor 204 that the set amount of time has passed.Microprocessor 204 determines the source of the interrupt and takes theappropriate action. For example, if a maximum time has elapsed since thelast heartbeat, microprocessor 204 might trigger atriumsensor/stimulator 212.

[0026] Microprocessor 204 preferably also monitors one or morephysiological signals. For example, microprocessor 204 may detectcardiac voltage signals via atrium sensor 212 and/or ventricle sensor214. Blood pressure, body temperature, and adaptive configuration datamay also be monitored. These signals preferably are logged in memory 206for later retrieval by programmer 110. Memory 206 preferably is largeenough to store a variety of physiological signals that are monitoredover a period of several days. This amount of data may comprise severalmegabytes of data. Memory 206 preferably is implemented as dynamicrandom access memory (DRAM) or other suitable memory type.

[0027] Atrium sensor/stimulator 212 is an interface circuit betweenmicroprocessor 204 and a heart lead coupled to an atrium of the heart.Similarly, ventricle sensor/stimulator 214 is an interface circuitbetween microprocessor 204 and a heart lead that is coupled to aventricle of the heart. These interface circuits are configured to applya customized electrical energy pulse to the respective region of theheart in response to a trigger signal from microprocessor 204. Interfacecircuits 212, 214 may also be configured to measure cardiac voltagesignals from the electrodes so that microprocessor 204 can monitor theperformance of the heart. The microprocessor 204 may store the cardiacwaveforms (or “electrograms”) in memory for subsequent retrieval by amedical technician.

[0028] Telemetry module 218 may be designed to be activated byprogrammer 110 when wand 108 enters into proximity with pacer 106. Thisevent causes telemetry module 218 to be activated and to notifymicroprocessor 204 of an incoming communication. Microprocessor 204monitors the incoming communication from programmer 110 and storesprogramming data or parameters, and responds to any requests. Forexample, one request might be to transfer the data from memory 206 toprogrammer 110. In this case, microprocessor 204 provides the data frommemory 206 to telemetry module 218 for transferal to programmer 110.

[0029]FIG. 3 is a schematic diagram of the communications channelsemployed by pacer 106 and programmer 110. A wand transmitter 302provides a communication signal which is transmitted to a pacer receiver304 through body tissues 306. This communication signal, for example,might represent a programmer request for the pacer 106 to transmit data.This technique using a pair of coils is well known to those of ordinaryskill in the art. An example of this technique is illustrated in U.S.Pat. No. 5,314,453, which is hereby incorporated by reference as thoughcompletely set forth herein.

[0030] To provide a download of a substantial amount of data in as shorta time as possible from pacer 106 to programmer 110, a high bandwidthconnection in the reverse direction (i.e. from the pacer to theprogrammer) is desired. This high-bandwidth connection comprises a pacertransmitter 308 which transmits a modulated light signal to a wandreceiver 310 through body tissues 306. It is contemplated that wandtransmitter 302 and implant receiver 304 are coils that communicate viaa shared inductive coupling. Thus one embodiment uses an inductivecoupling communications link for programmer 110 to transmit data andcommands to pacer 106, and an optical communications link to transmitdata and status information from pacer 106 to programmer 110.Alternatively, an optical link could be used to communicate in bothdirections.

[0031] It is contemplated that implant transmitter 308 includes an LEDthat emits light in the infrared (<4.3×10¹⁴ Hz), visible(4.3×10¹⁴-7.3×10¹⁴ Hz) or ultraviolet (>7.3×10¹⁴ Hz) frequency ranges,and that wand receiver 310 includes a light sensor sensitive to lightemitted by implant transmitter 308. The various frequencies (colors) oflight experience differing amounts of attenuation by body tissues 306.The light emitted by implant transmitter 308 preferably experiencesrelatively small losses while passing through body tissues 306.Experiments have been done using a light frequency of 5.42×10¹⁴ Hz(green light), but somewhat lower frequencies such as 4.3×10¹⁴-5.0×10¹⁴Hz may be preferred, and 4.5×10¹⁴-4.7×10¹⁴ Hz may be more preferred.

[0032]FIG. 4 is a block diagram of portions of one embodiment of aprogrammer 110. Programmer 110 includes a microprocessor 402, amodulator 404, a transmit coil 406, a light sensor 408, and ademodulator 410. Microprocessor 402 accepts and responds to user input(via controls not shown) and initiates communications with pacer 106.For example, if a user requests a download of data from pacer 106 toprogrammer 110, microprocessor 402 formulates a command signal, andsends the signal to modulator 404. Modulator 404 converts the commandsignal into a modulated signal for driving transmit coil 406. The signaldriving the transmit coil produces a changing magnetic field whichinduces a current in a receive coil in pacer 106. Pacer 106 processesthe induced current in a manner described further below. Pacer 106 cantransmit signals to programmer 110 by modulating a light signal. Themodulated light signal may be greatly attenuated by body tissues. Whenenabled, light sensor 408 detects and amplifies the modulated lightsignal to produce a detection signal. Demodulator 410 demodulates thedetection signal and converts it into the data transmitted by the pacer106. Demodulator 410 then provides the data to microprocessor 402 foreventual analysis and display.

[0033] Because the optical signal may be greatly attenuated (i.e.reduced in intensity) by body tissue, light sensor 408 preferably ishighly sensitive and must be protected from ambient light. Thisprotection may be provided in the form of an enable signal which isasserted only when the ambient light is blocked, e.g. when the wand ispressed flat against the torso. In one implementation, the enable signalmay be asserted when a mechanical switch is closed upon pressing thewand against the torso. In another implementation, the enable signal maybe asserted when a photo-transistor adjacent to the light sensor 408detects that the light intensity has fallen below a predeterminedthreshold.

[0034] One light sensor which is contemplated for use in wand 108 is aPMT (photo-multiplier tube) such as R5600-01 PMT from HamamatsuCorporation. PMT's are well known and widely available, and are able todetect single photons while maintaining a low noise level. This lightsensor is advantageously sensitive to light in the frequency range from4.3×10¹⁴ to 20.0×10¹⁴ Hz.

[0035] In another embodiment, light sensor 408 comprises a photo-diodewhich may be robust enough to withstand ambient light and sensitiveenough to detect attenuated light emissions from the pacer. This lightsensor advantageously does not require an enable signal and the meansfor generating the enable signal.

[0036]FIG. 5 shows a block diagram of an illustrative telemetry module218 of pacer 106. Telemetry module 218 comprises an implant receivercoil 502, a current sensor 504, a demodulator 506, a power converter508, a modulator 510, and a light source 512. A communication signalfrom wand 108 induces a current in coil 502. Current sensor 504 detectsthe induced currents and produces an amplified detection signalrepresentative of the communication signal sent by wand 108. Demodulator506 demodulates the communication signal to obtain the commands, dataand/or parameters being sent by wand 108. Microprocessor 204 processesthe demodulated signal and determines an appropriate response. Forexample, if the transmitted data represents a download request,microprocessor 204 will initiate a download of the requested data storedin memory 206, i.e. the microprocessor will cause data from memory 206to be supplied to modulator 510.

[0037] Referring still to FIG. 5, power converter 508 is coupled toimplant receiver coil 502 to convert the induced currents into storedenergy. As modulator 510 converts the data from microprocessor 204 intoa modulated signal, it uses stored energy from power converter 508 todrive light source 512 in accordance with the modulated signal. Lightsource 512 may be an LED (light emitting diode) which emits light with afrequency suitable to pass through the body to the wand. Preferably theLED emits light with a frequency between 4.3×10¹⁴ and 5.0×10¹⁴ Hz, butother frequencies may be used as well. The light emitted is modulated inaccordance with the modulated signal from modulator 510. The modulatedlight may be detected and demodulated by wand 108 to recover the datastored in memory 206 as described above.

[0038] In one embodiment, power converter 508 employs a full-waverectifier to convert the currents induced in coil 502 into aunidirectional charging current. The power converter also includes abank of switching capacitors to be charged by the unidirectionalcharging current and thereafter step up the voltage to charge an energystorage capacitor. Current sensor 504 may be configured to detect theinduced currents by sensing the voltage drop across one or more diodesin the full-wave rectifier.

[0039] In another embodiment, the LED is powered by power supply 202 ofpacer 106. Power converter 508 may be included for the purpose ofrecharging power supply 202.

[0040] Various modulation schemes may be employed for the communicationchannels. The wand-to-implant communications channel may use pulse-widthmodulation (PWM), frequency-shift keying (FSK), or other suitabletechniques. The implant-to-wand communications channel may also employany suitable techniques such as pulse-code modulation (PCM) and simplexsignaling. Both channels may employ channel coding for error detection,timing, and/or constraining power usage. Such channel coding techniquesare known to those of ordinary skill in the art. It is noted that lightsensor 408 may be configured to generate a detection signal which isproportional to the light intensity, and that consequently both digitaland analog amplitude modulation signaling is also supported by thecontemplated configuration.

[0041]FIG. 6 shows an exemplary configuration of wand 108 and pacer 106shown in cross-section. Wand 108 is pressed against body tissues 306proximate to the location of pacer 106 and in active communication withpacer 106. Pacer 106 comprises power supply 202, electronics module 602,implant receiver coil 502, light source 512, and header 604. Electronicsmodule 602 includes microprocessor 204, memory 206, timers 208, 210,sensor/stimulators 212, 214, current sensor 504, demodulator 506, powerconverter 508, and modulator 510. Electronics module 602 and thecomponents it contains may be constructed as a circuit board. Header 604is a transparent portion of pacer 106 which may include electricalconnectors for the heart leads (FIG. 2) and light source 512.Alternatively, light source 512 may be located in electronics module602. As electronics module 602 is normally placed in an opaque portionof pacer 106, light source 512 is configured to emit light in thedirection of the transparent header 604. A mirror may be located withinheader 604 to redirect the modulated light toward wand 108. This mirrormay be concave to reduce dispersion of the modulated light signal. Foreither placement of light source 512, header 604 may also have a portionof its exterior surface configured as a lens to reduce the dispersion ofthe modulated light signal. Some of these configurations are describedin U.S. Pat. No. 5,556,421, which is hereby incorporated by reference inits entirety.

[0042] Wand 108 illustratively comprises modulator 404, transmit coil406, light sensor 408, demodulator 410, ambient light detector 606,reflective surface 608, interface module 610, and user interface 612. Inone embodiment, light sensor 408 is placed near a convergence point oflight rays that reflect from reflective surface 608. Reflective surface608 is designed to increase the light-gathering ability of wand 108.Ambient light detector 606 is positioned within the concavity defined byreflective surface 608 and/or adjacent to light sensor 408. Ambientlight detector 606 provides the enable signal discussed in FIG. 4 when asensitive light sensor 408 is employed. Ambient light detector 606 maybe a photo-transistor or photo-diode or any other photo-sensitive devicerobust enough to withstand anticipated light levels when wand 108 isseparated from torso 102. Interface module 610 may be a linedriver/buffer for communications with the rest of programmer 110, andmay further comprise a power supply or converter for powering theelectronics of wand 108. User interface 612 may comprise buttons foruser input (e.g. begin download) and lights for user feedback regardingthe status of the communications link with the implanted device.Directional lights may also be provided to aid the user in positioningthe wand to achieve the highest communications signal-to-noise ratio andthe maximum communications rate for downloading information from thememory of the pacer.

[0043]FIG. 7 shows a second exemplary configuration of wand 108, inwhich mechanical switches 702 rather than ambient light detector 606 areused to provide the enable signal of FIG. 4. Mechanical switches 702 arepressure sensitive and positioned on the face of the wand so that whenthe wand is correctly pressed against the torso, the normally openswitches are all closed. Variations on this may be employed so long asthe enable signal is only asserted when the light sensor 408 is shieldedfrom excessive light levels. Numerous variations and modifications willbecome apparent to those skilled in the art once the above disclosure isfully appreciated. It is intended that the following claims beinterpreted to embrace all such variations and modifications.

What is claimed is:
 1. An implantable device capable of supporting ahigh-bandwidth optical communications link with an external device,wherein the implantable device comprises: a memory configured to storedata for later retrieval; a photo-emitter configured to generate lighthaving a transmission frequency in a frequency range from approximately4.3×10¹⁴ to approximately 20.0×10¹⁴ Hz; a modulator coupled to receivedata from the memory and configured to convert the data into anelectrical signal for driving the photo-emitter.
 2. The implantabledevice of claim 1 , wherein the transmission frequency is in a frequencyrange from approximately 4.3×10¹⁴ to approximately 7.3×10¹⁴Hz.
 3. Theimplantable device of claim 1 , wherein the transmission frequency is ina frequency range from approximately 4.5×10¹⁴ to approximately 4.7×10¹⁴Hz.
 4. The implantable device of claim 1 , further comprising: areceiver coil configured to generate an induced current in response to acommunication signal from the external programmer; a current sensorconfigured to detect the current induced in the receiver coil and toconvert the induced current into a detected signal; a demodulatorcoupled to the current sensor to receive the detected signal andconfigured to convert the detected signal into an operational signal forthe implantable device; and a microprocessor coupled to the demodulatorand the memory, wherein the microprocessor receives the operationalsignal from the demodulator, wherein the microprocessor collects datafor transmission to the programmer, and wherein the microprocessorstores the collected data in the memory.
 5. The implantable device ofclaim 4 , wherein the microprocessor is coupled to a stimulus generatorwhich operates in response to a trigger signal provided by themicroprocessor, and wherein the operational signal is used to determinetrigger signal characteristics.
 6. The implantable device of claim 4 ,further comprising a power converter coupled to the receiver coil andconfigured to convert the induced current into power to be supplied tothe modulator.
 7. The implantable device of claim 4 , further comprisinga power converter coupled to the receiver coil and the modulator toconvert induced current from the receiver coil into electrical power tooperate the photo-emitter.
 8. The implantable device of claim 1 ,further comprising a sensor configured to sample heart-generatedelectrical signals and coupled to the memory to provide the sampledsignals for storage.
 9. A system for transcutaneous communication,wherein the system comprises: an implantable device which includes: amemory configured to store data for later retrieval; a photo-emitterconfigured to generate light in response to a modulated signal; amodulator coupled to receive data from the memory and configured toconvert it into the modulated signal for driving the photo-emitter; anexternal unit which includes: a photo-multiplier configured to detectlight emitted by the photo-emitter and configured to responsivelygenerate a detection signal; a demodulator coupled to thephoto-multiplier to receive the detection signal and configured toconvert the detection signal into a data signal; a display coupled tothe demodulator to receive the data signal and configured to produce anoutput display representative of the data signal.
 10. The system ofclaim 9 , wherein the light produced by the photo-emitter has afrequency in a range from approximately 4.3×10¹⁴ to approximately20.0×10¹⁴ Hz.
 11. The system of claim 9 , wherein the external unitfurther includes: an external unit microprocessor configured to generatedata for storage in the memory; an external unit modulator coupled toreceive the data for storage from the external unit microprocessor andconfigured to convert the data into a communication signal; a signalingcoil connected proximate to the photo-multiplier and driven by thecommunication signal, wherein the signaling coil is configured toproduce a changing magnetic field.
 12. The system of claim 11 , whereinthe implantable device further includes: a receiver coil in which acurrent is induced that is representative of an induced currentrepresentative of the communication signal; a second demodulator coupledto receive the communication signal from the receiver coil andconfigured to convert the communication signal into the data forstorage, wherein the second demodulator is coupled to store the data inthe memory.
 13. The system of claim 12 , wherein the implantable devicefurther includes: a power converter coupled to the receiver coil toconvert the induced current into energy; a capacitor coupled to thepower converter to receive and store the energy, and configured tosupply the energy to the modulator for conversion into the modulatedsignal.
 14. A method for transcutaneous communication to an externaldevice, wherein the method comprises: retrieving stored data from amemory in an implanted device; converting the stored data into amodulation signal; applying the modulation signal to a photo-emitter toproduce light; positioning a photo-multiplier tube to detect the light;converting the light into a detection signal; and demodulating thedetection signal to reproduce the stored data.
 15. The method of claim14 , wherein the light produced by the photo-emitter has a frequency ina range from approximately 4.3×10¹⁴ to approximately 20.0×10¹⁴ Hz. 16.The method of claim 14 , further comprising: generating a programmingsignal in the external device; driving a signaling coil with theprogramming signal; inducing a current in a receiving coil in theimplanted device; converting the induced current into stored energy on acapacitor; using the stored energy to produce the modulation signal. 17.The method of claim 16 , further comprising: demodulating the inducedcurrent into program data; storing the program data in the memory in theimplanted device.